North Sea Sites Research Articles

Marine CSEM synthetic study to assess the detection of CO2 escape and saturation changes within a submarine chimney connected to a CO2 storage site

SUMMARY Carbon capture and storage (CCS) within sealed geological formations is an essential strategy to reduce global greenhouse gas emissions, the primary goal of the 2015 United Nations Paris Agreement. Large-scale commercial development of geological CO2 storage requires high-resolution remote sensing methods to monitor CO2 migration during/after injection. A geological formation containing a CO2 phase in its pore space commonly exhibits higher electrical resistivity than brine-saturated (background) sediments. Here, we explore the added value of the marine controlled-source electromagnetic (CSEM) method as an additional and relevant geophysical tool to monitor moderate to significant changes in CO2 saturation within a fluid conduit breaking through the seal of a CCS injection reservoir, using a suite of synthetic studies. Our 2-D CSEM synthetic models simulate various geological scenarios incorporating the main structural features and stratigraphy of two North Sea sites, the Scanner Pockmark and the Sleipner CCS site. Our results show significant differentiation of leakage through the seal with CO2 saturation ($S_{{\rm CO}_2}$) ranging between 20 and 50 per cent, while our rock physics model predicts that detection below 20 per cent would be challenging for CSEM alone. However, we are able to detect with our 2-D inversion models the effects of saturation with 10 and 20 per cent CO2 within a chimney with 10 per cent porosity. We demonstrate that simultaneous inversion of Ey and Ez synthetic electric field data facilitates a sharper delineation of a CO2 saturated chimney structure within the seal, whereas Ez synthetic data present higher sensitivity than Ey to $S_{{\rm CO}_2}$ variation, demonstrating the importance of acquiring the full 3-D electric field. This study illustrates the value of incorporating CSEM into measurement, monitoring and verification strategies for optimal operation of marine CCS sites.

The dynamic lives of osseous points from Late Palaeolithic/Early Mesolithic Doggerland: A detailed functional study of barbed and unbarbed points from the Dutch North Sea.

Osseous barbed and unbarbed points are commonly recovered from the Dutch North Sea and other Mesolithic sites of northern Europe. Interpreted as elements of projectile weaponry, barbed points are considered by archaeologists to be a technological innovation in the hunting equipment of hunter-gatherers. However, debate about their exact use and identification of the targeted prey species is still ongoing. To shed light on the function of these tools, we analysed a sample of 17 artefacts from the Netherlands with a multi-disciplinary approach encompassing morphometric, functional, and chemical analysis. 14C-AMS dating yielded the oldest date for a barbed point from the Dutch coast (⁓13000 cal. BP). The observation of microwear traces preserved on the tools provides solid evidence to interpret the function of barbed and unbarbed points. We show that there were two distinct tool categories. 1) Barbed points hafted with birch tar and animal or vegetal binding were likely projectile tips for terrestrial and aquatic hunting. We provide strong clues to support the link between small barbed points and fishing using wear traces. 2) Points without barbs served as perforators for animal hides. Our results highlight the importance of use-wear and residue analysis to reconstruct prehistoric hunting activities. The functional interpretation of projectile points must also rely on microwear traces and not merely on the association with faunal remains, historical sources, and ethnographic comparisons.

Investigation of marine temperature changes across temporal and spatial Gradients: Providing a fundament for studies on the effects of warming on marine ecosystem function and biodiversity

A current critical issue in climate change studies is how temperature changes and shifts on different spatial and temporal scales can affect organisms in terms of trends, variability and frequency of extremes. In this paper, we analysed marine temperature data on different temporal and spatial scales. We related the sea surface temperature data from the Helgoland Roads Time Series, one of the most important and detailed long-term in situ marine ecological time series, to the Sylt Roads, North Sea, Germany, Europe, North Atlantic and Northern Hemisphere surface temperatures. All time series showed a distinct upwards shift in temperature in the late 1980s, early 1990s, with positive trends in overall for the period between 1962 and 2019 ranging from 1 to 2 °C over 57 years. We quantified changes in temperature variability by comparing the years before and after 1990, on both long-term and seasonal scales. At Helgoland and Sylt, an increase in the number of warmer days in summer and a decrease in extremely cold days in winter are the new characteristics of the temperature pattern after 1990; higher than expected temperatures now also occur earlier during the year. For these locations, we observed the highest trends overall, i.e. of around 0.3 °C/decade. The observed bimodal shape of the probability density functions, characterized by winter and summer modes, had become more heterogeneous, with the cold mode peak moving to higher values and the steepness to the peak increasing, which is a consequence of a decrease in extremely cold days. North Atlantic Oscillation (NAO) and Multidecadal Oscillation (AMO) large-scale phenomena had no significant correlations or, for the NAO, were limited to the winter season at the regional and local scales. The closest landmass (mainland Germany) temperature was highly correlated with the North Sea sites. Taken together, our results suggest that marine pelagic ecosystems and their species are subject to temperature shifts with similar patterns but with variations in magnitude at the different scales. Temperature is one of the main drivers of species diversity and distribution, and this manifests on different spatial and temporal scales depending on population growth, life stages, cycles and habitat. Accordingly, we here present the temperature changes on the appropriate spatio-temporal scales, and thus provide the suitable and useful fundament for studies on the effects of warming on marine ecosystem function and biodiversity.

Characteristics of the low‐level jets observed over Dunkerque (North Sea French coast) using 4 years of wind lidar data

AbstractThis study investigates the occurrence and characteristics of low‐level jets (LLJs) up to 200 m above sea level in the North Sea area, specifically in the southernmost part of the basin, at the entrance of the English Channel. Using a short‐range Doppler lidar installed in Dunkerque Port, on the northern coast of France, wind profiles were recorded for 4 years and analyzed statistically. LLJs were detected on more than 11,000 of the 10‐min average wind profiles (5% of time), with similar jet core height and core speed distributions as in other sites in the southern North Sea area, and a similar annual cycle. However, there were differences in the core directions and the daily cycle, with afternoon northeastly jets being the most frequent in Dunkerque, whereas southwesterly nocturnal jets were dominant in the other North Sea sites. This suggests that wind channeling in the Dover Strait is likely a major factor for LLJ formation in Dunkerque region. The study also examined the conditions of LLJ occurrence using European Centre for Medium‐Range Weather Forecast (ECMWF) reanalysis v5 and ultrasonic anemometer data, but the different types of LLJs sharing the same core direction and occurrence conditions made it difficult to quantify specific jet formation mechanisms. This is the first experimental study on the North Sea shore, and at the entrance of the English Channel, providing valuable insights into the LLJ behavior in the region.

The influence of the drainage regime on the installation and the response to vertical cyclic loading of suction caissons in dense sand

Offshore wind turbines are considered as an integral part of renewable energy. Dense sand often characterises the sea bed and suction caisson foundations are increasingly used to support offshore wind turbines at North Sea sites. Significant experience with this technology exists in the oil and gas industry, although these platforms are typically located in deeper water with predominantly clay soils. Therefore, research is required to transfer the existing knowledge. This paper discusses a series of centrifuge tests that investigate the installation and the load transfer mechanisms governing the response of suction caissons embedded in dense sand subjected to vertical cyclic loading. The effect of potential loosening of the internal soil plug during suction installation on the in-service foundation performance is evaluated through cone penetration tests inside the caisson following suction installation at different pumping flow rates. In accompanying tests, the load displacement behaviour in response to vertical cyclic loading is investigated, including significant excursions into tension. The results identify the pore fluid behaviour as the key factor affecting the response, with the pumping flow rate applied during installation being less influential. Based on the findings from the experiments a simplified approach for the prediction of suction caisson behaviour under vertical cyclic loading is presented.

Chemicals of emerging concern in marine specimens of the German Environmental Specimen Bank

BackgroundDescriptor 8 of the Marine Strategy Framework Directive (MSFD) (Directive 2008/56/EC) addresses the good environmental status with regard to pollution of marine waters by chemical contaminants. Commission decision (EU) 2017/848 lays down the criteria and methodological standards on good environmental status of marine waters. Member States, in regional or subregional cooperation, shall establish lists of relevant contaminants beside those already covered by the Water Framework Directive (WFD). To provide information on emerging contaminants in marine biota, the German Environmental Specimen Bank (ESB) has compiled data of blue mussels and eelpouts from coastal sites in the North and Baltic Seas. Substances identified by the International Council for the Exploration of the Sea (ICES) as of emerging concern for the marine environment have been used as a starting point.Results and conclusionsThe study presents data of 19 emerging flame retardants and degradation products, 40 per- and polyfluoroalkyl substances (PFAS) and three cyclic volatile methylsiloxanes (cVMS). Among the emerging flame retardants, only Dec 602 was detected in all samples of 2015. Dec 604, Cl10-antiDP, 1,5-DPMA, EH-TBB, PBEB, TBP-AE, BATE, BTBPE and HBBz were constantly < limit of quantification (LOQ). Time trends were barely detected. Legacy PBDE still dominates in most samples. PFAS concentrations were usually higher in samples from the North Sea sites compared to samples from the Baltic Sea. PFOS dominated in most samples. Increasing trends over time were detected for PFNA, PFDA and PFDoDA at the Baltic Sea site and for PFDA at one North Sea site. Concentrations of the cVMS D4, D5 and D6 were below the detection limit at the ESB sampling sites. Based on the results, it should be considered to include the emerging flame retardants DP and Dec 602 and the long-chain perfluoroalkyl substances PFNA, PFDA, PFUnDA and PFDoDA in a regular monitoring in the North and Baltic Seas.

Seasonal variability in metal and metalloid burdens of mussels: using data from the German Environmental Specimen Bank to evaluate implications for long-term mussel monitoring programs

BackgroundMetal and metalloid concentrations in mussels can vary between seasons. In biota monitoring, the sampling time is therefore an important issue. Within the German Environmental Specimen Bank (ESB) program blue mussels (Mytilus edulis Complex) are sampled regularly since the 1980s. The samples are collected in two-monthly intervals at two North Sea sites and in 6-month intervals at one Baltic Sea site. All samples from one site and year are combined to annual composite samples and archived as sub-samples under cryogenic conditions. In order to investigate a possible reduction of the number of annual sampling intervals while maintaining comparability with the long-term composite sample data, the seasonal variability of metals/metalloids was analyzed based on the half-yearly and bimonthly samples of 2013, 2015 and 2017.ResultsIn mussels from the North Sea site Eckwarderhoerne seasonality of metals/metalloids was comparable in all 3 years (arsenic being the only exception). At the North Sea site Koenigshafen seasonality of cobalt, nickel, cadmium, copper, lead, and arsenic was comparable in 2013 and 2015 but not in 2017, while selenium showed the same seasonality in all 3 years. Within 1 year, concentrations of metals and metalloids can vary by the same order of magnitude as observed between annual composite samples of different years making it impossible to select just one representative sampling time point per year that would provide the same information as the respective annual composite sample.ConclusionsThe findings highlight the importance of carefully selecting the sampling time point when using mussels in biota monitoring. For the German ESB program it is recommended to continue with the current sampling strategy and analyze annual composite samples in order to maintain comparability with the long-term data series, which are a special feature of the ESB.

Empirical modelling for derived metrics as function of sound exposure level in marine pile driving

To protect marine life, piling activities at sea are subject to regulations on sound emission. Different regulatory authorities base different action criteria based on different statistics of the emitted sound pressure. Modelling efforts to predict these metrics have achieved mixed success. While the sound exposure level (SEL) can be predicted relatively well, it has proven harder to model the peak sound pressure level (Lpk) accurately. It would therefore be valuable to have a reliable way of estimating Lpk, based on a prediction of SEL. Correlations between SEL and Lpk and between SEL and rms sound pressure level (Lrms) are obtained from measurements during piling of the Luchterduinen wind farm off the Dutch west coast, and are used to assess the applicability of correlations found at other wind farms. A metastudy using data from Luchterduinen as well as the German Bight gives a more robust trend line for use at other North Sea sites. Lrms is computed in two different ways, involving the 90 % energy signal duration on the one hand and the effective signal time duration (Teff) on the other. The value of Lrms based on Teff is found to be more robust. [Work sponsored by BOEM.]

Underwater noise levels in UK waters.

Underwater noise from human activities appears to be rising, with ramifications for acoustically sensitive marine organisms and the functioning of marine ecosystems. Policymakers are beginning to address the risk of ecological impact, but are constrained by a lack of data on current and historic noise levels. Here, we present the first nationally coordinated effort to quantify underwater noise levels, in support of UK policy objectives under the EU Marine Strategy Framework Directive (MSFD). Field measurements were made during 2013–2014 at twelve sites around the UK. Median noise levels ranged from 81.5–95.5 dB re 1 μPa for one-third octave bands from 63–500 Hz. Noise exposure varied considerably, with little anthropogenic influence at the Celtic Sea site, to several North Sea sites with persistent vessel noise. Comparison of acoustic metrics found that the RMS level (conventionally used to represent the mean) was highly skewed by outliers, exceeding the 97th percentile at some frequencies. We conclude that environmental indicators of anthropogenic noise should instead use percentiles, to ensure statistical robustness. Power analysis indicated that at least three decades of continuous monitoring would be required to detect trends of similar magnitude to historic rises in noise levels observed in the Northeast Pacific.

Validation of SeaWiFS and MODIS Aqua/Terra aerosol products in coastal regions of European marginal seas

The aerosol products associated with the ocean colour missions SeaWiFS and MODIS (both Aqua and Terra) are assessed with AERONET field measurements collected in four European marginal seas for which fairly large uncertainties in ocean colour in-water products have been documented: the northern Adriatic, the Baltic, Black and North Seas. On average, more than 500 match-ups are found for each basin and satellite mission, showing an overall consistency of validation statistics across the three missions. The median absolute relative difference between satellite and field values of aerosol optical thickness τa at 443nm varies from 12% to 15% for the three missions at the northern Adriatic and Black Sea sites, and from 13% to 26% for the Baltic and North Sea sites. It is in the interval 16–31% for the near-infrared band. The spectral shape of τa is well reproduced with a median bias of the Ångström exponent varying between -15% and +14%, which represents a clear improvement with respect to previous versions of the atmospheric correction scheme. These results show that the uncertainty associated with τa in the considered coastal waters of the European marginal seas is comparable to global validation statistics.

Processes controlling the dynamics of compound sand waves in the North Sea, Netherlands

The understanding of the morphodynamics of harmonic bed forms on the seabed is essential for modeling marine sediment transport and coastal morphologic development. Previous research has mainly focused on the type and distribution of bed forms, but areally extensive data and time series of seabed features are scarce. Multibeam and side‐scan sonar data from four expeditions reveal the contrasts between a coastal site with asymmetric and flattened, three‐dimensional (3‐D) compound sand waves on a shoreface‐connected ridge and an offshore site with asymmetric and sharp‐crested, 2‐D compound sand waves. Migration rates of the coastal sand waves are 6.5–20 m yr−1, while migration rates of the offshore sand waves are −3.6 to 10 m yr−1. This contrasting morphology and dynamic behavior of compound sand waves at the two North Sea sites is explained by differences in the relative importance of tidal currents and wave activity near the bed. These new field data provide parameters and boundary conditions for sand transport models, while the empirically derived behavior of sand waves may be used to validate sand transport and sand wave models.

Resource degradation: a subtle effect of bottom fishing

Populations of hermit crabs are critically limited by the availability of suitable gastropod shells that they utilise to reduce their risk of predation and environmental stress. Common whelks are the main source of shells for large hermit crabs in the northern Atlantic but are vulnerable to direct and indirect effects of fishing activity. This study examined the potential consequences of degrading shell resources for common hermit crabs. Laboratory trials demonstrated that hermit crabs avoid low-quality damaged shells throughout their life history. This laboratory preference was corroborated by direct field observations of shells preferentially occupied by hermit crabs, compared with shells available for occupation. In the field, 8 times as many empty shells had holes compared to shells occupied by hermit crabs. In the North Sea, the abundance and biomass of live whelks and hermit crabs collected at sites where they co-occurred were significantly related. However, whelks occurred at far fewer sites overall and were more patchily distributed at high abundance than hermit crabs, which were more widespread. At a subset of sites, whelks of the same body-mass range occurred in the Irish and North Sea. However, at these sites, hermit crabs sampled from the North Sea had a significantly lower biomass. This suggests that the shells available for occupation at the North Sea sites would not support crabs of a body mass comparable to that found in the Irish Sea. Using published data, we calculated that in some of the intensively fished areas of the North Sea, 24% of the available shell resource will be damaged each year. The reduction in shell quality in the North Sea may impose a physical constraint on the upper size limit currently attainable by hermit crabs and hence may have implications for population viability.

EPR investigations of Mn 2+, Fe 3+ ions and carbonaceous radicals in atmospheric particulate aerosols during their transport over the eastern coast of the English Channel

Electron paramagnetic resonance (EPR) spectroscopy was used to follow the evolution of the chemical forms of manganese and study other paramagnetic species in the atmospheric particulate aerosols at Wimereux, a French station located on the eastern coast of the English Channel. In parallel, Graphite Furnace Atomic Absorption Spectrometry (GFAAS) was used to measure the metal concentrations. Fe concentration values are common in North Sea sites, but those of Mn are quite higher due to the presence of an important local source of Mn. EPR spectra have evidenced isolated Fe 3+ and Mn 2+ ions and carbonaceous products with variable intensities according to the wind directions. Amounts of paramagnetic species and carbonaceous products are maximum for continental winds and minimum for marine winds. Three Mn types were identified depending on the sampling distance from the source of emission and the size of the particles.

Differential response of macrozoobenthos to marine sand extraction in the North Sea and the Western Mediterranean

Sublittoral benthic coastal communities of the North Sea and of the Western Mediterranean were studied before and after sand extraction between 1993 and 1995 at borrow sites in Denmark, The Netherlands, and Spain. Recolonization of disturbed areas was fast owing to the rapid increase of opportunistic species. At the North Sea sites, the benthic community largely recovered within 2–4 years, whereas in Spain recovery is expected to take longer. The response of zoobenthos to sand extraction is discussed, taking into account differences in site characteristics, extraction methods, and recovery time of the habitats. The effects on the benthic community appear to be related to the physical impact on the sea floor. Small-scale disturbances in seabed morphology and sediment composition result in short-term effects on the benthic community. However, larger disturbances mainly caused by sediment composition may have a prolonged effect, particularly in low dynamic systems such as those present in the Mediterranean.

Trace element levels in fish from clean and polluted coastal marine sites in the Mediterranean Sea, Red Sea and North Sea

The bioaccumulation of Hg, Cd, Zn, Cu, Mn and Fe was evaluated in the muscle and liver tissue of four fish species (Siganus rivulatus, Diplodus sargus, Lithognatus mormyrus and Plathychtis flesus) from clean and polluted marine coastal sites in the Red Sea, Mediterranean Sea and North Sea within the framework of the MARS 1 program. Representative liver samples were screened for organic contaminants (DDE, PCBs and PAHs) which exhibited very low concentrations. The levels of Cd, Cu, Zn, Fe and Mn found in the muscle tissue in this study were similar among the four species and within the naturally occurring metal ranges. However, differences were found among the sites. In the Red Sea, Cu was higher in the muscle of S. rivulatus at Ardag and Zn at the Observatory (OBS). Cu, Zn and Mn were higher in the Red Sea than in the specimens from the Mediterranean. The differences were attributed to different diets derived from distinctively different natural environments. D. sargus from Haifa Bay (HB) had higher Cd, Cu and Mn values than specimens from Jaffa (JFA), and L. mormyrus higher Cd, Fe and Mn in HB, corresponding to the polluted environmental status of the Bay. No differences in metal levels were found among the North Sea sites, except for Fe that was lower at the Eider station. Hg was low in all the specimens, but the values varied with species and sites. The lowest Hg values were found in S. rivulatus, the herbivorous species, as expected from its trophic level. Hg in P. flesus was higher than in S. rivulatus but still low. Higher Hg values were found in the muscle tissue of L. mormyrus,with the highest values in D. sargus, both carnivorous species from the same family. Hg in D. sargus was higher in HB than in JFA, as expected, but in the larger specimens of L. mormyrus from JFA values were higher, while in the small specimens there were no differences in Hg values. The levels of all metals were higher in the liver than in the muscle, with enrichment factors ranging from 3 to 104, depending on species and sites. The lowest enrichment values were found for Hg. Based on liver values, the specimens of S. rivulatus from the OBS had the highest levels, as well as D. sargus and L. mormyrus from JFA, contrary to the known relative environmental status of the sites.

Decadal variability on the northwest european shelf

The seasonal cycles of sea surface temperature and other parameters at eight sites on the Northwest European Shelf show an interannual variability in the range of years to decades for the years 1960–1995. Statistical methods like spectral analysis or phase diagrams prove that only a few frequency peaks beyond the annual cycle are of real significance. Most prominent signals for temperature and salinity on the shelf are the periods around 8 and 17 years. In general the time series are too short to identify definitely long-term trends or transitions between different states of equilibrium. However, there is an indication of a shift in the thermohaline regime along the Belgian-Dutch coast from the eighties to the nineties. Explanations for the decadal variability can be given by the intrinsic time scales of the internal shelf dynamics as well as by external periods. A spectral analysis of calculated mass fluxes at sections across the Northern and the Channel entrances of the North Sea show significant peaks beyond the seasonal cycle.This suggests an advective influence from the Atlantic Ocean. Another external source of decadal variability on the Northwest European Shelf is the atmosphere. Here no phase shift can be expected between corresponding signals of time series at different North Sea sites as it is indeed observed. The article is substantially a description and analysis of decadal variability based on observational and computational data as well as on statistical methods complied in the «Northwest European Shelf Programme” (NOWESP). The respective details are explained in other papers of this volume.

Use of soil fabric studies in evaluating the strengths of clays in a North Sea site investigation : Marsland, A; Love, M; Loken, T; Lunne, T Q J Engng GeolV23, N2, 1990, P125–146

Abstract Soil fabric features influence both the large-scale engineering properties and measurements made in laboratory and field tests. The effects on the large-scale parameters and different types of tests can vary appreciably. In the absence of relevant full-scale test data from the site being investigated, fabric studies can play a vital role in the evaluation of design parameters from laboratory and small in situ tests. This is especially the case where relationships between test and large scale behaviour have previously been obtained in soils with similar fabric features and stress histories. The potential value of fabric studies is particularly important in offshore investigations where large-scale in situ tests are not feasible and even the data from laboratory and small in situ tests are severely limited. During a major investigation for a gravity platform in the Norwegian sector of the North Sea in 1981 it was found that the relationships between the results obtained from the different types of tests did not fit into the patterns previously found at other North Sea sites. The need to explain these anomalous results and the availability of adequate numbers of good quality samples provided an excellent opportunity to make extensive observations and measurements of the soil fabric features and to use them in the interpretation of the test data and the evaluation of design parameters. This paper describes the detailed macro-and micro-fabric studies which were made and their role in explaining the anomalies in the test data and in the evaluation of the design parameters.

The use of soil fabric studies in evaluating the strengths of clays in a North Sea site investigation

Abstract Soil fabric features influence both the large-scale engineering properties and measurements made in laboratory and field tests. The effects on the large-scale parameters and different types of tests can vary appreciably. In the absence of relevant full-scale test data from the site being investigated, fabric studies can play a vital role in the evaluation of design parameters from laboratory and small in situ tests. This is especially the case where relationships between test and large scale behaviour have previously been obtained in soils with similar fabric features and stress histories. The potential value of fabric studies is particularly important in offshore investigations where large-scale in situ tests are not feasible and even the data from laboratory and small in situ tests are severely limited. During a major investigation for a gravity platform in the Norwegian sector of the North Sea in 1981 it was found that the relationships between the results obtained from the different types of tests did not fit into the patterns previously found at other North Sea sites. The need to explain these anomalous results and the availability of adequate numbers of good quality samples provided an excellent opportunity to make extensive observations and measurements of the soil fabric features and to use them in the interpretation of the test data and the evaluation of design parameters. This paper describes the detailed macro-and micro-fabric studies which were made and their role in explaining the anomalies in the test data and in the evaluation of the design parameters.

Polymorphism of mannose phosphate isomerase in North Sea and Baltic Sea populations of the amphipods Gammarus zaddachi and G. salinus

Genetic variation at the mannosephosphate isomerase (MPI) locus was examined in the euryhaline sibling species Gammarus zaddachi Sexton and G. salinus Spooner. Both crustacean amphipods share identical enzyme mobilities, following electrophoresis on vertical starch gels. The MPI locus turned out to be highly polymorphic; it is encoded by 6 alleles in G. zaddachi and 7 alleles in G. salinus. Geographic variation of allelic diversity was studied in samples from 9 G. zaddachi and 10 G. salinus populations, primarily obtained from Baltic Sea and North Sea sites. Patterns of inter- and intraspecific heterogeneity are described. Differences in allelic composition exist between Baltic and North Sea samples of G. salinus. In G. zaddachi, levels of polymorphism are higher in North Sea populations than in those from Baltic Sea areas. The significance of these findings is discussed in the light of previous biochemical genetic investigations on the population structure of the two amphipods considered.

Genetic variation between geographic populations of the amphipods Gammarus zaddachi and G. salinus

Spatial and temporal patterns of gene-enzyme variation were estimated in the sibling species Gammarus zaddachi Sexton and G. salinus Spooner by starch gel electrophoresis. Twenty-one G. zaddachi and 18 G. salinus populations from coastal and estuarine areas in the Baltic Sea, North Sea and other localities of north-western Europe were surveyed. Both amphipods display similar electrophoresis patterns of the enzyme systems studied. Considerable interspecific and interpopulational differences were detected in allele frequencies at three highly polymorphic loci, phosphoglucose isomerase (PGI), glutamate oxalacetate transaminase (GOT) and arginine phosphokinase (APK). G. zaddachi exhibits a pronounced genetic heterogeneity in most areas of the sampled range. Populations from northern French and western English coasts differ significantly from the other samples in allele frequencies at the PGI or APK locus, respectively. Baltic populations are widely uniform in their genetic composition but can be distinguished from samples taken at North Sea sites in allele frequencies at the APK locus. The latter reveal a clinal variation, ranging from the Danish to the French coast. In contrast to G. zaddachi, a low degree of genetic differentiation was observed among the G. salinus populations examined. This indicates that migration and interregional mixing may be more important in maintaining the genetic structure than in G. zaddachi which, compared to G. salinus, prefers habitats of lower salinity levels. Evidently, less extensive dispersal capabilities owing to the confinement of G. zaddachi to brackish waters of dilute salt concentrations may account for a diminished gene flow and considerable genetic separation of local populations. This assumption is supported by the genetic homogeneity documented in Baltic G. zaddachi populations. In view of the low and constant salinities in wide areas of this brackish-water sea such barriers do not exist. Survey studies performed with selected populations over a 3-yr period demonstrated a general pattern of temporal constancy in the allozyme variation observed.